Treatment of mica for electrical purposes



Patented Feb. 15, 1938 UNITED STATES ATENT OFFICE.

Frank William Brough,

Chiswick, London, England, assignor to Electric & Musical Industries Limited, Hayes, England, a company of Great Britain No Drawing. Applicati No. 15,819. In Great Britain 1 Claim.

The present invention relates to insulating ma terial for use in electric discharge devices.

It is well known that mica, when used as insulating material in discharge devices such as 5 thermionic valves, is liable to display erratic insulating properties. The variations in leakage which are experienced are thought to be due to adsorbed films from the metals and other materials within the device, to breakdown of the 0 surface resulting in conducting layers and to carbonization of dirt that may have been left on the mica, particularly on the edges thereof.

It is an object of the present invention to provide a method of treating mica whereby its insulating properties may be improved.

According to the present invention there is provided a method of improving the insulating properties of mica characterized by the fact that the mica is heated to a temperature at which its thickness increases markedly. The mica may thereafter be subjected to pressure in such a manner as to reduce its thickness, and it may then be used as an insulator in an electric discharge device.

In carrying the invention into effect, mica is stoved in hydrogen, air or in vacuo to such a temperature that its thickness becomes up to 10 times its original value. It is then very fragile, and to increase its strength it is pressed until 30 it becomes firm and coherent. The pressing opon April 11, 1935, Serial June 8, 1934 eration may be carried out after the mica has cooled. The mica after the above treatment has a shiny metallic appearance and gives off little gas when the discharge device in which it is used is pumped. After the pressing operation the mica is not as firm and strong as before the stoving treatment but it is sufiiciently robustfor many purposes in connection with the interior of discharge devices and its insulating properties are greatly improved.

The temperature at which the best stoving is obtained has been found to lie between 800 and 1000 C. Below 800 C. the mica does not become suficiently degassed. If the mica is stoved above 1000 (3., the mica becomes very brittle and tends to break up into powder when pressed. The time of stoving is from half a minute upwards to five minutes.

I claim:

A method of treating mica for use in electron discharge devices prior to its insertion into an electron discharge device which method comprises heating the mica to a temperature between 800 C. and 1000 C. for a period of at least half a minute and at most five minutes, said period being so chosen that a desired increase of thickness results, and thereafter subjecting said mica to pressure, thereby permanently reducing its thickness.

FRANK WILLIAM BROUGH. 

